The most severe deficiency of composite pad products is the splitting of the web upon minor amounts of strain, which causes the underlying layers of absorbed material to disintegrate and fall out of the diaper or pad structure, which can cause irritation of the skin and other discomfort. A stretchable web overcomes these problems.
Several spun-bonded processes, especially those using a "melt-blowing" concept, are known in the prior art such as Hartmann, U.S. Pat. No. 3,502,763 and Butin, U.S. Pat. No. 3,849,241, which show the production of melt-blown webs of fine enough filament size, but the webs are lacking in one or more of the essential and critical properties, like bond strength of the filament crossover points or yield to stress (splittiness). In the present invention, these deficiencies are overcome by a unique combination of process parameters and material properties that achieves the desired balance of the required web properties which make it suitable as top liner for diapers, pads, and the like.
Stretchability is generally achieved by low molecular orientation of a filament as e.g., measured by its birefringence. Low birefringence is usually accompanied with relatively slow spinning speeds (see Hartmann, U.S. Pat. No. 3,502,763). At high spinning speeds, usually high birefringence and strength, accompanied with low residual elongation of the filaments is obtained, which in many applications is preferred. To obtain economically practical spinning rates as well as the desired low molecular orientation, it is necessary to spin at extremely low melt-viscosity requiring high temperatures and using very low molecular weight polymers. High spinning rates and filament velocities are furthermore required in order to form strong bonds of filament crossover points caused by impact of the filaments on the collecting surface, where the filaments are still tacky when the web is formed. This method, however, produces a web of low birefringence but high crystallinity; the high crystallinity in turn causes brittleness in low molecular weight fibers such as used in this process, which causes the undesirable splittiness.